[FAB-9035] Clean up membership.md
[FAB-9035] Clean up membership.md

Fix typos, clean up language.

Change-Id: I6eeacabf59784493f179a403bbe1abc76d6fd8a3
Signed-off-by: joe-alewine <fe2d55ddcdde6f951d073228e1f2dc081e79a271@ibm.com>

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